To add a preheader text, you'll have to switch to "Code View" and add the following code to the very top of your email copy.
<div style="font-size:0px;line-height:1px;mso-line-height-rule:exactly;display:none;max-width:0px;max-height:0px;opacity:0;overflow:hidden;mso-hide:all;">This is a pre-header text.</div>
Just change the "This is a pre-header text" part with just what you want to show in the preview, and then switch out of code view.
You won't see the preheader text when previewing the email on Instantly, but when you send yourself a test email, you'll see that the preheader part shows up in preview, but not when you open the actual email.